1. Field of the Invention
This invention relates in general to interferometric techniques for surface characterization. In particular, it relates to a method for measuring the thickness of a layer deposited over a substrate, such as a diamond-like carbon layer over a magnetic head.
2. Description of the Related Art
In the manufacture of read/write magnetic-head sliders, precise and rapid profilometry is essential for quality control purposes. The precise height of the various slider components is critical to ensure performance and long product life. As illustrated schematically in the top view and cross-section of FIGS. 1(A) and 1(B), respectively, magnetic-head sliders include an air-bearing surface 10 (ABS) made of an aluminum-oxide/titanium-carbide composite material (often referred to as AlTiC), a read/write pole-tip region 12 (which can be made of several different materials), and a trailing-edge surface 14 made of aluminum oxide. The working distance between the air bearing surface of the slider and the disk surface affects the potential for a mechanical crash as the head flies over the disk. Similarly, the distance between the pole tip and the disk affects signal loss during read/write operations.
Therefore, standard tests carried out for quality control during manufacture of head sliders involve the measurement of the difference between the heights of the ABS surface 10 and the trailing-edge surface 14 (commonly referred to as the aluminum oxide trailing-edge recession, or ALR, parameter) and of the distance between the heights of the ABS surface 10 and the pole tip 12 (commonly referred to as the pole tip recession, or PTR, parameter). The composite ABS surface 10 is precision polished in order to render it as flat as possible for optimal functionality. Thus, the height of the ABS surface is conveniently identified for the purpose of calculating the ALR and PTR parameters by fitting a plane surface 16 to the height data obtained by means of an interferometric measurement of a predetermined ABS region. However, the composite structure and the corresponding granularity of the AlTiC material tend to produce imprecise height measurements by conventional interferometry.
Copending U.S. Ser. No. 11/824,127, hereby incorporated by reference in its entirety, discloses a novel approach to account for interferometric-measurement errors introduced by composite structures in general and AlTiC material of ABS surfaces in particular. The method involves performing an initial determination of the surface profile of the composite material using a conventional interferometric approach. The composition of the mixture constituting the composite material (Al2O3 and TiC, for example) is calculated at every pixel of the surface using an empirical relation between modulation and the absolute value of reflectivity and by assuming a linear relationship between the composite reflectivity of the composite material and the theoretical reflectivity of each constituent. An absolute value of reflectivity for the composite material is obtained from the empirical relation and the modulation measured while profiling the sample surface.
The concentration of each constituent of the composite material is then determined from the absolute value of reflectivity for the composite material and the theoretical values of reflectivity of its constituents using the assumed linear relationship. The linear relationship is subsequently also used to calculate an effective composite (complex) reflectivity for the composite material from the concentration and the theoretical reflectivity of each constituent. The phase change on reflection (PCOR), δij, at each pixel can thus be determined using the conventional theoretical relationship between phase change and reflectivity. Once the PCOR is calculated for each pixel, the initial surface profile is corrected by adding the corresponding fraction of wavelength to the height of each pixel.
In the more specific case of ALR- and PTR-parameter calculation for magnetic-head sliders, the plane surface fitted to the profile of the ABS (AlTiC) surface is corrected according to the invention. The recessions between the ABS surface and the aluminum-oxide trailing edge and between the ABS surface and the pole tip can thus be refined using a corrected reference plane calculated from actual reflectance data. This approach produces an order of magnitude improvement in the calculation of ALR and PTR parameters.
However, magnetic-head sliders are typically coated with a protective diamond-like carbon layer (referred to as a DLC layer in the art). FIG. 1(C) illustrates a head slider wherein all regions are coated with such a DLC layer of uniform thickness L. Thus, all interferometric measurements are necessarily carried out through the thin-film of DLC, which has to be accounted for in the calculation of the ALR and PTR parameters. Ser. No. 11/824,127 teaches a correction for the DLC layer when its thickness L is either known or can be assumed with some degree of precision. However, no interferometric technique is known for PCOR correction when the thickness of the DLC layer in not known with some degree of certainty, which is usually the case.
Therefore, there is still a need for an interferometric technique that allows for PCOR correction when the thickness of the DLC layer is unknown. This invention provides a straightforward solution to that problem.